The invention concerns a friction engagement device, in particular a brake or a clutch for motor vehicles. The friction engagement device contains at least one first component and at least one second component, that can move relative to each other, in that, as a rule, one of the components rotates and the other component remains at rest. The first component is provided with at least one contact surface and consists, at least in its region bordering on the contact surface, of a material with high heat conductivity, such as aluminum or an aluminum alloy. The second component contains a support component to which at least one friction lining is applied with adhesive, riveting or the like. In order to perform a friction engagement for braking or engaging a clutch, the friction surface of the friction lining can be pressed against the contact surface.
The rapid removal of heat from the region of friction contact during the deceleration (braking, clutch engagement) is critically important for the durability and load capacity of a brake or clutch of a motor vehicle, for example, a tractor or passenger vehicle, in order to keep the peak temperature below a specific critical temperature at which the brake or clutch could be damaged and lose its effectiveness or even fail completely.
Due to increasing operating speeds, particularly in the case of tractors, ever greater amounts of braking energy are applied to service and auxiliary brakes in the form of heat. In view of the increasing load density and the associated temperature problems, the durability of the components, particularly the brake lining, is increasingly endangered or its life reduced.
Commonly steel or cast iron are applied in brakes as opposing material to the friction linings of brake bands or opposing disks. This leads to poor heat flow from the friction contact zone and to great weight which is largely undesirable, in particular in future light-weight vehicle concepts.
Beyond that, multi-component brake drums are known, in which a friction ring as a separate component is fastened, fixed against rotation, to the radially inner side of a pot-shaped brake drum. In that way US-A-3,005,259 discloses a brake drum of aluminum with a friction ring of a cast iron material, where the friction ring is fastened to the brake drum, fixed against rotation, by means of radial undercuts. For reasons of weight reduction the brake drum is made of aluminum. But the friction ring consists of a cast iron material and cannot guarantee a rapid flow of heat from the friction contact surface.
EP-A-0 879 975 describes a brake drum component group, consisting of a pot-shaped brake drum of aluminum, that is fastened, fixed against rotation, to the hub of a vehicle axle, as well as a friction ring of metal matrix composite material (MMC) fastened, fixed against rotation, to the brake drum. Due to the MMC material these brake drum component groups enjoy a very high load capacity and thermal resistance. Since the brake drum consists of aluminum it also provides advantages in weight reduction. Due to the use of the MMC material, however, these brake drum component groups are very costly and complicated to manufacture.
Furthermore DE-A-42 43 516 describes a brake lining for brakes with aluminum brake disks. In order to avoid strength problems in the aluminum brake disks at temperatures above 450xc2x0 C., the friction material is to be selected in such a way that its friction coefficient remains constant up to a temperature of 400xc2x0 C., and declines at higher temperatures. However, brakes of this configuration are recommended only for relatively low braking forces.
U.S. Pat. No. 5,339,931 describes a friction material made of phenolic resin; an organic friction modifier such as rubber scrap, cellulose, latex, cork, or cashew particles; an aramid fiber; a carbonaceous material such as graphite, synthetic graphite, carbon and coke; glass fiber; inorganic friction modifiers such as barytes, whiting, talc, rottenstone, vermiculite or suzorite mica; abrasive particles such as silica, magnesia, zircon, zirconia, mullite, alumina or iron oxides; a lubricant, and 2-18 percent porous copper powder. This friction material is specially formulated for use with aluminum metal matrix composites and can operate at temperatures that approach 450xc2x0 C.
The problem underlying the invention is seen as that of defining a friction engagement device of the type defined above, that is able to absorb high braking energy levels, exhibits high durability life together with very good heat conductivity out of the friction contact zone, can be manufactured cost effectively and is sufficiently rigid to absorb and transmit high forces and torques.
It has been found, in particular for friction engagement devices in which high energy levels are encountered, that the combination of aluminum or aluminum alloy contact surfaces on the one hand, with friction linings of an organic material on the other hand, can fulfill the requirements imposed. The power level absorbed by the friction engagement device can be increased considerably compared to previous devices. On the one hand, the increase in the peak temperature in the friction contact zone is reduced by approximately 30% by the use of aluminum or an aluminum alloy in place of the otherwise usual materials such as steel and cast iron. Thereby the friction engagement device can absorb a higher load during a friction engagement, for example, during braking, or its durability is clearly increased at a given load. This permits a limited-time durability to be converted into a long-term durability. On the other hand, cost effective but temperature sensitive organic materials that could burn in the case of poor heat conduction can be applied without their susceptibility to high temperature being detrimental. Increased size friction contact surfaces and a resulting larger space requirement and increased weight for the friction engagement device can be avoided.
Heavy, high speed utility vehicles, particularly agricultural tractors, that are being designed for vehicle speeds of 50 km/h and above, require powerful brakes. As a rule, generally there is only a relatively small space available for this purpose, so that the brakes must be designed for high power densities. For example, with such tractors the parking brake (hand brake) should not exceed an average thermal density of 3 W/mm2. In the case of the service brake (foot brake) this value is closer to 1 W/mm2. These values can be exceeded with a friction engagement device according to the invention.
As used herein for the present invention, the friction material is predominantly made of carbon based compounds. A preferred formulation uses mainly cellulosic fibers mixed with synthetic thermally stable fibers such as Kevlar(trademark) or graphite fibers bound in a resin. For reasons of cost, it is advantageous to generally use paper linings for friction linings. For example, a friction lining of the type HM200E of the company Miba, Austria, is appropriate for a high static and dynamic friction value. The friction coefficient of the friction lining is appropriately greater than 0.1.
In dry brakes, as a rule, abraded material of the lining appears in the contact zone. The abraded material can become embedded in the friction contact surface, particularly in the aluminum surface, and increase its roughness, which can lead to increased wear and finally to the destruction of the friction surface and the friction lining. According to a particularly preferred further development of the invention, these wear phenomena can be effectively avoided by immersing the rotating first or second component at least partially in a lubricating oil sump, so that the friction surface is moistened constantly.
The lubricant here preferably is a lubricating oil, particularly a usual hydraulic oil, a brake oil or a gear oil. Many vehicles already contain lubricant circulating systems. The lubrication of the friction engagement device can be incorporated into these circulating systems to great advantage. It is appropriate that the lubricant conducted in a circulating system also be cooled in the usual manner.
According to a further preferred embodiment of the invention, an aluminum alloy is applied here with a thermal conductivity that is greater than 120 W/mxc2x0K, preferably greater than 150 W/mxc2x0K. The aluminum alloy preferably contains between 5-14% silicon and between 0-5% copper. For example, an appropriate aluminum alloy is AlSi9Cu3 where the thermal conductivity is greater than that of the usual iron brake components and guarantees a rapid heat flux away from the friction surface.
Particularly with friction engagement devices that are run dry, it may be advantageous to subject the contact surface (friction surface) that consists of aluminum or an aluminum alloy of the first component to a surface treatment by hard anodizing. Thereby the surface is strengthened within one layer, in a thickness of a few xcexcm, so that in any case the lining wears, but not the aluminum contact surface.
With friction engagement devices that are operated wet, this measure can usually be omitted.
It is particularly advantageous to apply the solution according to the invention to brakes with brake drums, where the first component that consists of aluminum or an aluminum alloy is the brake drum. This brake may be an auxiliary brake (hand brake) that is used for an emergency retarding of the vehicle and for its parking. Here a brake band, which carries the friction lining of organic material, is slung about the brake drum with its good heat conductivity. The brake drum is arranged in a housing which is partially filled with gear oil so that the brake drum is running in oil and the friction surface is constantly moistened.
Since the layers of material located closest to the friction contact zone are generally, critical to the rapid transfer of heat, the brake drum can be configured as a (two-piece) composite component, in that the supporting structure is manufactured of steel or cast iron that is paired, fixed against rotation, with a friction element (friction ring) of aluminum. Thereby the strength and cost advantages of steel/cast iron as opposed to aluminum or aluminum alloy can be utilized in connection with the better heat transfer of the aluminum materials. The friction lining in the brake band consists of organic material, for example, paper.
A further, particularly preferred area of application is the service brake. Here the second component is a rotating full brake disk, that carries at least one brake lining on both sides in its radially outer region. The brake disks are associated with metal rings arranged, fixed against rotation, of which at least one can be moved axially in order to establish the friction engagement with the brake linings. The metal rings consist of aluminum or an aluminum alloy.
The invention can be applied analogously in place of a service brake configured as a disk brake, also as a multiple-disk brake or a multiple-disk clutch.
The drawing shows embodiments of the invention on the basis of which the invention as well as further advantages and advantageous further developments and embodiments of the invention shall be explained and described in greater detail in the following.